The production of a liposome which encapsulates a macromolecule by high-pressure extrusion process is known to the art. An article by Peter I. Lelkes discloses the use of a French press to form various forms of encapsulating liposomes, for example, a homogeneous population of small unilamellar vesicles, i.e. liposomes, or a mixture of oligolamellar and multilamellar vesicles, all referred to as FPVs. See Liposome Technology, Gregory Gregoriadis (ed.), Vol. 1, Chap. 5, (1984) pp. 51-65. Essentially, an aqueous suspension of a hydrated phospholipid encapsulating a macromolecule is extruded through an orifice at pressures of between 5,000 and 20,000 psi. If the extrusion is done at a temperature above the phase transition temperature, then small unilamellar liposomes result. However, if not, then a mixture of oligolamellar and multilamellar liposomes are formed.
One of the uses for liposome with encapsulated macromolecules is as a label for diagnostic assays. For example, in U.S. Pat. No. 4,342,826, issued to Francis X. Cole, a liposome is filled with an enzyme and is conjugated to an antibody to form a labelled reactant in a homogeneous immunoassay. After an immunocomplex is formed between the antibody and the corresponding antigen, a signal is generated when complement, a specific mixture of serum proteins, fixes to the surface of the immunocomplex and creates holes in the lipid bilayer wall of the liposome and allow substrate on the outside of the liposome to be exposed to the enzyme on the inside. In a preferred embodiment the substrate undergoes a colorimetric change upon exposure. An important limitation to this assay is the type of liposome used to encapsulate the enzyme. Multilamellar liposomes have upwards of ten phospholipid bilayers surrounding the enzyme, an undesirable barrier to complement. Only unilamellar or oligolamellar liposomes are suitable for use in immunoassays.